E.I. du Pont de Nemours and Company (DuPont) has been making plexifilaments or plexifilamentary film-fibril webs in a flash spinning process for a number of years. DuPont has obtained numerous patents describing the process and equipment including U.S. Pat. No. 3,081,519 to Blades et al., U.S. Pat. No. 3,169,899 to Steuber, U.S. Pat. No. 3,227,794 to Anderson et al., U.S. Pat. No. 3,484,899 to Smith, U.S. Pat. No. 3,497,918 to Pollock et al., U.S. Pat. No. 3,860,369 to Brethauer et al., U.S. Pat. No. 4,352,650 to Marshall, U.S. Pat. No. 4,554,207 to Lee, and U.S. Pat. No. 5,123,983 to Marshall.
The term "plexifilamentary" as used herein, means a three-dimensional integral network of a multitude of thin, ribbon-like, film-fibril elements of random length and with a mean film thickness of less than about 4 microns and a median fibril width of less than about 25 microns. In plexifilamentary structures, the film-fibril elements are generally coextensively aligned with the longitudinal axis of the structure and they intermittently unite and separate at irregular intervals in various places throughout the length, width and thickness of the structure to form a continuous three-dimensional network.
Referring now to FIG. 1 of the drawings, a spin cell 10 is shown with a large chamber 11 in which a fiber web W is flash spun and formed into a sheet S. The illustration of the spin cell 10 is quite schematic and fragmentary for purposes of explanation. A schematically illustrated spinpack, generally indicated by the number 12, is positioned within the chamber 11 of the spin cell 10 and is in the process of spinning the fiber web W. It should be understood that the process of manufacturing plexifilamentary sheet material includes the use of a multiple spinpacks similar to spinpack 12 which are arranged in the spin cell 10 for spinning and laying down other webs W with edges that overlap each other to form a wide sheet.
The spinpack 12 spins the web from a polymer solution which is provided to the spinpack 12 through a conduit 20. The polymer solution is provided at high temperature and pressure so as to be a single phase solution. The polymer solution is then admitted through a letdown orifice 22 into a letdown chamber 24. There is a pressure drop through the letdown orifice 22 so that the solution experiences a slightly lower pressure in the letdown chamber. At this lower pressure, the single phase solution becomes a two phase solution. A first phase of the two phase solution has a relatively higher concentration of polymer as compared to the polymer concentration of the second phase which has a relatively lower concentration of polymer. The system operates such that concentration of polymer in the solution in conduit 20 may be anywhere from slightly less than ten percent to in excess of twenty five percent based on weight and depending on the spin agent used. Thus, the polymer rich phase may still have more spin agent than polymer on a comparative weight basis. Based on observations, the polymer rich phase appears to be the continuous phase.
From the letdown chamber 24, the two phase polymer solution exits through a spin orifice 26 and enters the spin cell 10 which is at a much lower temperature and pressure than the letdown chamber 24. At such a low pressure and temperature, the spin agent evaporates or flashes from the polymer such that the polymer is immediately formed into plexifilamentary film-fibrils. The plexifilamentary film-fibrils exit the spin orifice 26 at very high velocity and are formed into a flattened web W by impacting a baffle 29. The baffle 29 further redirects the flattened web along a path that is roughly 90 degrees relative to the axis of the spin orifice (generally downwardly in the FIG. 1). The baffle 29, as described in other DuPont patents such as those noted above, rotates at high speed and has a surface contour to cause the web W to oscillate in a back and forth motion in the widthwise direction of the conveyor belt 15.
On the conveyor belt 15, the sheet has the form of a batt of fibers very loosely attached together. The batt is run under a nip roller 16 to consolidate the batt into the sheet product S which is then wound up on roll 17. The sheet product S is then taken to a finishing facility where it may be subjected to an assortment of processes depending on the desired end use for the sheet material. For example, the sheet product S may be fully bonded to make TYVEK.RTM. sheet material for envelopes and housewrap. TYVEK.RTM. is a registered trademark of DuPont. Fully bonded sheet is formed from the sheet product S by pressing it on heated rolls. The heat is maintained at a predetermined temperature (depending on the desired characteristics of the final sheet product) such that the web bonds together under pressure to form a sheet that has substantial strength and toughness while maintaining its opaque quality.
One of the concerns when running a flash spinning system is maintaining the pressure and temperature of the solution at desired levels as the polymer moves to the spin orifice. As the spin agent evaporates at the spin orifice 26, the spin orifice and its local environment are subject to evaporative cooling. To counteract the loss of heat at the spin orifice, steam is provided to circulate within the spinpack 12. As shown in FIG. 2, the spin block 30 of the conventional spinpack 12 is made of high strength stainless steel and includes steam channels 31 to maintain a desired temperature for the polymer solution and to provide heat to both the spin orifice 26 and the nozzle 33 adjacent the spin orifice 26. The nose of the spin head is made of copper which is more conductive of heat than the steel which comprises the majority of the remainder of the spinpack. This copper nose cone 35 is able to efficiently conduct heat from the steam channels of the spin block 30 to the spin orifice. The copper nose cone 35 and the spin block 30 are together jointly referred to herein as the spin head of the spin pack 12.
DuPont is instituting a new flash spinning system as part of a change from using a chlorofluorocarbon ("CFC") spin agent to using other hydrocarbon-based spin agents. While the considerations involved in selecting a spin agent are complicated and outside the scope of the present invention, it is noted that the spinpacks used in the past with the CFC spin agent were very large and provided flexibility in that the structure was large and accessible. For a variety of reasons, the spinpacks most useful with hydrocarbon-based spin agents are far smaller and the elements of such spinpacks must be smaller so as to find a place on the new spinpacks.
The spinpack historically used with CFC spin agents includes a copper nose cone 35 with internal threads (not shown in FIG. 2) arranged to mate with the external threads on a spin block 30. However, in a smaller spinpack, there is much less copper material in the nose cone available to conduct heat from the spinblock to the spin orifice at a rate sufficient to counteract evaporative cooling of the spin orifice. When the size of the spin block is significantly reduced, the resulting dimension of the nose cone 35 becomes too small to conduct sufficient heat to the spin orifice 26 and nozzle 33. The small dimension is of particular concern at the position indicated by the arrows 36 in FIG. 2.
Accordingly, a new means for connecting the copper nose cone to the spin block is needed that provides for improved heat transfer between the spin block and copper nose cone. This new connecting means must function well even though the thermal expansion coefficient for the copper in the nose cone is significantly different than the thermal expansion coefficient for the steel of the remainder of the spinpack. It must be recognized that the dimensions of the copper and steel parts of the spin head will change at different rates as the temperature of the spin head changes from the temperature at which the spinpack is assembled to the operating temperature. The new connecting means must not become loose when the nose cone expands more than the spin block due to their differing rates of thermal expansion. As spinpack reliability is a very important concern for continuous operation of a flash spinning system, any connection problem that could cause a shutdown of a spinpack cannot be tolerated.
Thus, a spinpack is needed that overcomes the above noted drawbacks and provides a nose cone design that will resist loosening during spin cell operation.
More specifically, a nose cone is needed that may expand but will effectively maintain a tight fit against the spin block supporting structure even when the spin block does not expand to the same extent as the nose cone.
The present invention provides an attachment between a first element and a second element wherein the first element has a different and greater coefficient of thermal expansion than said second element. The attachment arrangement includes outwardly directed screw threads on the first element and inwardly directed screw threads on the second element suited for engaging with outwardly directed screw threads. The second element further includes a shoulder which is positioned circumferentially outwardly from said internal threads of the second element. The first element includes a contact surface arranged to contact the shoulder when the first element is threadedly attached and secured to the second element. The shoulder is positioned even with or longitudinally back from the last of the inter-engaged threads between the first and second elements.